Hitherto, a so-called molybdenum-bismuth-iron mixed oxide catalyst is used for preparing unsaturated aldehydes and/or unsaturated carboxylic acids by a gas phase catalytic oxidation reaction of a corresponding raw material compound. However, the catalytic activity of the catalyst decreases when the catalyst is used for the oxidation reaction for a long time or when it receives excessive thermal load. Therefore, it is highly desired to provide a process for restoring the once decreased catalytic activity of such a catalyst to regenerate the catalyst which can be well used in the catalytic oxidation reaction.
As a process for regenerating a catalyst used for preparing unsaturated aldehydes and/or unsaturated carboxylic acids, EP 0 169 449 A2, EP 0 339 119 A1 and JP 05-184945 A disclose processes comprising thermally treating deteriorated catalysts in an atmosphere of a gas containing molecular oxygen, and U.S. Pat. No. 4,425,255 discloses a process comprising thermally treating a deteriorated catalyst in an atmosphere of a reducing gas at a temperature of 200 to 700° C. and then thermally treating the catalyst in an atmosphere of a gas containing molecular oxygen at a temperature of 550 to 700° C.
However, the catalysts which are regenerated by the above processes tend to have an excessively high oxidizing ability on raw material compounds, and to have lowered selectivities. In addition, the duration of the catalytic activities of such catalysts is shortened and the catalysts should be regenerated at short intervals. Therefore, the processes of the prior art are not always satisfactory.